The present invention relates to cellular telephone systems. More specifically, the present invention relates to base station transmit power control during handover situations in a code division multiple access cellular telephone system.
In a cellular communications system, mobile radio stations communicate over respective radio channels to a base station. Several base stations are connected to a switching node which is typically connected to a gateway that interfaces the cellular communications system with other communications systems. A call placed from an external network to a mobile station is directed to the gateway, and from the gateway, through one or more switching nodes to the base station(s) which serve(s) the called mobile station. A base station pages the called mobile station and establishes a radio communications channel.
In a Code Division Multiple Access (CDMA) mobile communications system, the information transmitted between a base station and mobile station is modulated by a mathematical code, sometimes called a spreading code, in order to distinguish that information from information associated with other mobile stations or base stations that are using the same radio frequency band. Accordingly, individual radio xe2x80x9cchannelsxe2x80x9d are discriminated on the basis of these codes. Various aspects of CDMA are set forth in one or more textbooks such as Applications of CDMA and Wireless/Personal Communications, Garg, Vijay K. et al., Prentice-Hall (1997). Spread spectrum communications permit mobile transmissions to be received at two or more (xe2x80x9cdiversexe2x80x9d) base stations and processed simultaneously to generate one received signal. With these combined signal processing capabilities, it is possible to perform a handover from one base station to another, (or from one antenna sector to another sector on the same base station), without any perceptible disturbance in the voice or data communications. This kind of handover is typically called xe2x80x9cdiversity handoverxe2x80x9d. During diversity handover, the signaling and voice information from plural base stations is combined in a common point with decisions being made on the xe2x80x9cqualityxe2x80x9d of the received data. Typically this common point is located at the switching node connected to the base stations. In the opposite direction, voice and signaling information are transmit from plural base stations, and a mobile station combines the results.
CDMA systems provide xe2x80x9csoftxe2x80x9d and xe2x80x9csofterxe2x80x9d diversity handover. In soft handover, as the mobile station moves to the edge of a cell, the adjacent cell""s base station assigns a transceiver to the call while the current base station continues to handle the call. As a result, the call is handled by both base stations on a make-before-break basis. The soft diversity handover occurs with both base stations handling the call until the mobile station moves sufficiently close to one of the base stations which then exclusively handles the call. Softer diversity handover occurs when the mobile station is in handover between two different sectors at the same base station.
Because all users in a CDMA communications system transmit information using the same frequency band at the same time, each user""s communications interferes with communications of the other users. In addition, signals received by a base station from a mobile station close to the base station are much stronger than signals received from other mobile stations located at the base station cell boundary. As a result, distant mobile communications are overshadowed and dominated by close-in mobile stations which is why this condition is sometimes referred to as the xe2x80x9cnear-far effect.xe2x80x9d
Therefore, to achieve increased capacity by decreasing unnecessary interference, all mobile-transmitted signals should arrive at a base station with about the same average power irrespective of their distance from the base station. Accordingly, uplink (or reverse) transmit power control (TPC) from the mobile station to the base station is one of the most significant factors in improving the performance and capacity of a CDMA system. In general, the mobile station attempts to control its transmit power based on the signal strength to generate a signal-to-noise (SNR) value (or other suitable measure) of signals received from a base station (open loop transmit power control), and the base station sends transmit power control messages to the mobile station (closed loop power control) with the end goal to control the power received at the base station to within a relatively small tolerance, e.g., 1 dB, for all mobile station transmissions received at that base station.
Downlink (or forward) transmit power control is also important for transmissions from the base station to the mobile station. Specifically, the base station varies its transmit power depending upon downlink transmit power control messages or commands sent by the mobile station. There are several reasons for downlink power control.
One reason for downlink transmit power control is to accommodate the fact that in certain coverage area locations, the downlink channel from base station to mobile station may be unusually poor. An example of such a location is a point where the path loss to one or two neighboring cells is nearly the same as the path loss to the base station communicating with the mobile station. At that location, the total interference is increased several times over the interference experienced by a mobile station at a point relatively close to the base station. An additional reason is that interference from these neighboring cell sites will not fade in unison with the desired signal. Still further, the mobile station may be located where several strong multipath signals arrive resulting in a relatively large interference. In other situations, the mobile station may be located where the signal-to-interference ratio is unusually good. Another reason for downlink power control is to minimize intercell interference caused by unnecessarily high base station transmit power levels. By varying the downlink power level to the minimum value needed to achieve a particular quality, unnecessary interference is avoided. If the downlink power level is fixed at a constant but high level to ensure minimum quality under poor conditions, much of the time the downlink power would be too high thereby causing needless interference. Such interference reduces cell capacity. In all of these cases, it is advantageous for the base station to raise or lower its transmit power to ensure acceptable quality but at the same time reduce to the extent possible interference to other signals.
Because power control in CDMA systems is important, transmit power control adjustments occur very frequently, e.g., every 0.625 milliseconds. In adjusting downlink transmit power, the mobile station is continually measuring the transmit power level received from the base station and determining whether that measured value is higher than a reference value. If so, one or more transmit power control bits having one value are transmit uplink from the mobile station to the base station to decrease the transmit power by a predetermined increment, e.g., 1 dB, down to a minimum transmit power value. On the other hand, when the measured value is lower than the reference value, one or more opposite value transmit power control bits are transmitted uplink to the base station to increase the transmit power by a predetermined increment, e.g., 1 dB, up to a maximum value. This transmit power control begins while uplink and downlink synchronization are being acquired and continues throughout the communication.
A problem arises in coordinating downlink transmit power levels among base stations at diversity handover. One problem is that during a handover, the transmit power command from the mobile station to the base stations involved in the handover may be erroneously received at one or more of the base stations. Another problem is that an offset between the downlink transmit powers of base stations involved in the soft handover can be quite large. Offset refers to the difference between the transmit power level of one (or more) base stations currently serving the mobile station and the power level of a new base station engaged in diversity handover with the mobile station. While an offset may or may not be desired for a particular application, it is difficult in either situation for the new base station to know the old base station""s transmit power level when the new base station starts to transmit. Because transmit power is adjusted so rapidly, e.g., sixteen adjustments every 10 milliseconds, by the time a transmit power level is determined and delivered to the new/target handover base station based on a TPC command, the actual transmit power at one or more of the old serving base stations already serving the mobile station may have significantly changed during that time delay.
Consider the following scenario. At the beginning of a soft handover, transmit power level measurements from a serving base station and from a target base station to a mobile station are sent to a network node. The network node reads these transmit power measurement reports periodically, determines an appropriate power setting for the target base stationxe2x80x94either the same power or a power at a specific desired offset, and sends the determined power setting to the target base station.
By the time that the target base station receives the determined transmit power from the network node, 100 msec up to one second of time may have transpired. During that one second, the serving base station transmit power level could have changed one hundred or even a thousand times because of the speed at which transmit power is adjusted, e.g., every 0.625 milliseconds. Hence, the determined transmit power command for the target base station is likely considerably offset from what that transmit power level should be at the time the command is actually received and implemented at the target base station, i.e., after a significant time delay.
What is needed is a way in which to compensate for or otherwise eliminate this undesired base station transmit power offset at handover. Moreover, the solution to the above-identified problem should preferably be a simple one that allows the mobile station to only transmit one set of power control commands to all of the base stations involved in the handover.
It is an object of the present invention to overcome the problems identified above, and in particular, to provide base station transmit power coordination technique that does not increase the complexity of the mobile station.
It is another object of the invention to eliminate erroneous transmit power settings including undesirable offset transmit power settings at base stations involved in a diversity handover.
It is a further object of the invention to compensate for drifting in the transmit power levels between base stations involved in diversity handover. For example, as a result of different error rates on the uplink control channels over which the mobile station transmits the same downlink power control command to these base stations, each base station ends up transmitting more or less different power levels depending on the respective bit error rates.
A power control method in accordance with the present invention synchronizes the transmit power levels at the base stations currently serving a mobile station and an initial transmit power level of a new target base station being added in a diversity handover situation. At the beginning of the handover, a radio network controller orders the serving base stations to detect their respective transmit powers to the mobile station. Each serving base station reports its transmit power (to the mobile station) to the radio network controller. The radio network controller determines an initial transmit power setting for the target base station and new transmit power settings for the serving base stations synchronized to a particular time (t0). The initial and new transmit powers and the synchronizing time are provided to the respective base stations involved in the soft handover. The target base station transmits to the mobile station at the initial power setting, and the serving base stations adjust their transmit powers toward the new values. In a preferred, example embodiment of the present invention, the serving base station power adjustment is performed gradually with the target base station transmitting at the initial power setting and the serving base stations adjusting their transmit powers toward the new values at the synchronizing time t0.
In another further example embodiment, a future synchronizing time may be employed and provided to all base stations to coordinate the transmit power levels so that desired values are reached at that synchronizing time. The invention may also be employed to periodically correct for drift in previously coordinated base station transmit power levels. Still further, the invention may be employed to control transmit power levels of different base station sectors involved in a softer handover.